Disruptor—核心源码实现分析（三）
4.Disruptor的高性能原因
一.使用了环形结构 + 数组 + 内存预加载
二.使用了单线程写的方式并配合内存屏障
三.消除伪共享(填充缓存行)
四.序号栅栏和序号配合使用来消除锁
五.提供了多种不同性能的等待策略
5.Disruptor高性能之数据结构(内存预加载机制)
(1)RingBuffer使用环形数组来存储元素
环形数组可以避免数组扩容和缩容带来的性能损耗。
(2)RingBuffer采用了内存预加载机制
初始化RingBuffer时，会将entries数组里的每一个元素都先new出来。比如RingBuffer的大小设置为8，那么初始化RingBuffer时，就会先将entries数组的8个元素分别指向新new出来的空的Event对象。往RingBuffer填充元素时，只是将对应的Event对象进行赋值。所以RingBuffer中的Event对象是一直存活着的，也就是说它能最小程度减少系统GC频率，从而提升性能。
public class Main {public static void main(String[] args) {//参数准备OrderEventFactory orderEventFactory = new OrderEventFactory();int ringBufferSize = 4;ExecutorService executor = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());//参数一：eventFactory，消息(Event)工厂对象//参数二：ringBufferSize，容器的长度//参数三：executor，线程池(建议使用自定义线程池)，RejectedExecutionHandler//参数四：ProducerType，单生产者还是多生产者//参数五：waitStrategy，等待策略//1.实例化Disruptor对象Disruptor<OrderEvent> disruptor = new Disruptor<OrderEvent>(orderEventFactory,ringBufferSize,executor,ProducerType.SINGLE,new BlockingWaitStrategy());//2.添加Event处理器，用于处理事件//也就是构建Disruptor与消费者的一个关联关系disruptor.handleEventsWith(new OrderEventHandler());//3.启动disruptordisruptor.start();//4.获取实际存储数据的容器: RingBufferRingBuffer<OrderEvent> ringBuffer = disruptor.getRingBuffer();OrderEventProducer producer = new OrderEventProducer(ringBuffer);ByteBuffer bb = ByteBuffer.allocate(8);for (long i = 0; i < 5; i++) {bb.putLong(0, i);//向容器中投递数据producer.sendData(bb);}disruptor.shutdown();executor.shutdown();}
}public class Disruptor<T> {private final RingBuffer<T> ringBuffer;private final Executor executor;...//Create a new Disruptor.//@param eventFactory the factory to create events in the ring buffer.//@param ringBufferSize the size of the ring buffer, must be power of 2.//@param executor an Executor to execute event processors.//@param producerType the claim strategy to use for the ring buffer.//@param waitStrategy the wait strategy to use for the ring buffer.public Disruptor(final EventFactory<T> eventFactory, final int ringBufferSize, final Executor executor, final ProducerType producerType, final WaitStrategy waitStrategy) {this(RingBuffer.create(producerType, eventFactory, ringBufferSize, waitStrategy), executor);}//Private constructor helperprivate Disruptor(final RingBuffer<T> ringBuffer, final Executor executor) {this.ringBuffer = ringBuffer;this.executor = executor;}...
}//Ring based store of reusable entries containing the data representing an event being exchanged between event producer and EventProcessors.
//@param <E> implementation storing the data for sharing during exchange or parallel coordination of an event.
public final class RingBuffer<E> extends RingBufferFields<E> implements Cursored, EventSequencer<E>, EventSink<E> {//值为-1public static final long INITIAL_CURSOR_VALUE = Sequence.INITIAL_VALUE;protected long p1, p2, p3, p4, p5, p6, p7;...//Create a new Ring Buffer with the specified producer type (SINGLE or MULTI)public static <E> RingBuffer<E> create(ProducerType producerType, EventFactory<E> factory, int bufferSize, WaitStrategy waitStrategy) {switch (producerType) {case SINGLE:return createSingleProducer(factory, bufferSize, waitStrategy);case MULTI:return createMultiProducer(factory, bufferSize, waitStrategy);default:throw new IllegalStateException(producerType.toString());}}//Create a new single producer RingBuffer with the specified wait strategy.public static <E> RingBuffer<E> createSingleProducer(EventFactory<E> factory, int bufferSize, WaitStrategy waitStrategy) {SingleProducerSequencer sequencer = new SingleProducerSequencer(bufferSize, waitStrategy);return new RingBuffer<E>(factory, sequencer);}//Construct a RingBuffer with the full option set.//@param eventFactory to newInstance entries for filling the RingBuffer//@param sequencer sequencer to handle the ordering of events moving through the RingBuffer.RingBuffer(EventFactory<E> eventFactory, Sequencer sequencer) {super(eventFactory, sequencer);}...
}abstract class RingBufferFields<E> extends RingBufferPad {private final long indexMask;//环形数组存储事件消息private final Object[] entries;protected final int bufferSize;//RingBuffer的sequencer属性代表了当前线程对应的生产者protected final Sequencer sequencer;...RingBufferFields(EventFactory<E> eventFactory, Sequencer sequencer) {this.sequencer = sequencer;this.bufferSize = sequencer.getBufferSize();if (bufferSize < 1) {throw new IllegalArgumentException("bufferSize must not be less than 1");}if (Integer.bitCount(bufferSize) != 1) {throw new IllegalArgumentException("bufferSize must be a power of 2");}this.indexMask = bufferSize - 1;//初始化数组this.entries = new Object[sequencer.getBufferSize() + 2 * BUFFER_PAD];//内存预加载fill(eventFactory);}private void fill(EventFactory<E> eventFactory) {for (int i = 0; i < bufferSize; i++) {//设置一个空的数据对象entries[BUFFER_PAD + i] = eventFactory.newInstance();}}...
}abstract class RingBufferPad {protected long p1, p2, p3, p4, p5, p6, p7;
}6.Disruptor高性能之内核(使用单线程写)
Disruptor的RingBuffer之所以可以做到完全无锁是因为单线程写。离开单线程写，没有任何技术可以做到完全无锁。Redis和Netty等高性能技术框架也是利用单线程写来实现的。
具体就是：单生产者时，固然只有一个生产者线程在写。多生产者时，每个生产者线程都只会写各自获取到的Sequence序号对应的环形数组的元素，从而使得多个生产者线程相互之间不会产生写冲突。
7.Disruptor高性能之系统内存优化(内存屏障)
要正确实现无锁，还需要另外一个关键技术——内存屏障。对应到Java语言，就是valotile变量与Happens Before语义。
内存屏障：Linux的smp_wmb()/smp_rmb()。
8.Disruptor高性能之系统缓存优化(消除伪共享)
CPU缓存是以缓存行(Cache Line)为单位进行存储的。缓存行是2的整数幂个连续字节，一般为32-256个字节，最常见的缓存行大小是64个字节。
当多线程修改互相独立的变量时，如果这些变量共享同一个缓存行，就会对这个缓存行形成竞争，从而无意中影响彼此性能，这就是伪共享。
消除伪共享：利用了空间换时间的思想。
由于代表着一个序号的Sequence其核心字段value是一个long型变量(占8个字节)，所以有可能会出现多个Sequence对象的value变量共享同一个缓存行。因此，需要对Sequence对象的value变量消除伪共享。具体做法就是：对Sequence对象的value变量前后增加7个long型变量。
注意：伪共享与Sequence的静态变量无关，因为静态变量本身就是多个线程共享的，而不是多个线程隔离独立的。
class LhsPadding {protected long p1, p2, p3, p4, p5, p6, p7;
}class Value extends LhsPadding {protected volatile long value;
}class RhsPadding extends Value {protected long p9, p10, p11, p12, p13, p14, p15;
}public class Sequence extends RhsPadding {static final long INITIAL_VALUE = -1L;private static final Unsafe UNSAFE;private static final long VALUE_OFFSET;static {UNSAFE = Util.getUnsafe();try {VALUE_OFFSET = UNSAFE.objectFieldOffset(Value.class.getDeclaredField("value"));} catch (final Exception e) {throw new RuntimeException(e);}}//Create a sequence initialised to -1.public Sequence() {this(INITIAL_VALUE);}//Create a sequence with a specified initial value.public Sequence(final long initialValue) {UNSAFE.putOrderedLong(this, VALUE_OFFSET, initialValue);}//Perform a volatile read of this sequence's value.public long get() {return value;}//Perform an ordered write of this sequence. //The intent is a Store/Store barrier between this write and any previous store.public void set(final long value) {UNSAFE.putOrderedLong(this, VALUE_OFFSET, value);}...
}9.Disruptor高性能之序号获取优化(自旋 + CAS)
生产者投递Event时会使用"long sequence = ringBuffer.next()"获取序号，而序号栅栏SequenceBarrier和会序号Sequence搭配起来一起使用，用来协调和管理消费者和生产者的工作节奏，避免锁的使用。
各个消费者和生产者都持有自己的序号，这些序号需满足如下条件以避免生产者速度过快，将还没来得及消费的消息覆盖。
一.消费者序号数值必须小于生产者序号数值
二.消费者序号数值必须小于其前置消费者的序号数值
三.生产者序号数值不能大于消费者中最小的序号数值高性能的序号获取优化：为避免生产者每次执行next()获取序号时，都要查询消费者的最小序号，Disruptor采取了自旋 + LockSupport挂起线程 + 缓存最小序号 + CAS来优化。既避免了锁，也尽量在不耗费CPU的情况下提升了性能。
单生产者的情况下，只有一个线程添加元素，此时没必要使用锁。多生产者的情况下，会有多个线程并发获取Sequence序号添加元素，此时会通过自旋 + CAS避免锁。
public class OrderEventProducer {private RingBuffer<OrderEvent> ringBuffer;public OrderEventProducer(RingBuffer<OrderEvent> ringBuffer) {this.ringBuffer = ringBuffer;}public void sendData(ByteBuffer data) {//1.在生产者发送消息时, 首先需要从ringBuffer里获取一个可用的序号long sequence = ringBuffer.next();try {//2.根据这个序号, 找到具体的"OrderEvent"元素//注意：此时获取的OrderEvent对象是一个没有被赋值的"空对象"OrderEvent event = ringBuffer.get(sequence);//3.进行实际的赋值处理event.setValue(data.getLong(0));} finally {//4.提交发布操作ringBuffer.publish(sequence);}}
}//Ring based store of reusable entries containing the data representing an event being exchanged between event producer and EventProcessors.
//@param <E> implementation storing the data for sharing during exchange or parallel coordination of an event.
public final class RingBuffer<E> extends RingBufferFields<E> implements Cursored, EventSequencer<E>, EventSink<E> {//值为-1public static final long INITIAL_CURSOR_VALUE = Sequence.INITIAL_VALUE;protected long p1, p2, p3, p4, p5, p6, p7;...//Increment and return the next sequence for the ring buffer.//Calls of this method should ensure that they always publish the sequence afterward.//E.g.// long sequence = ringBuffer.next();// try {// Event e = ringBuffer.get(sequence);// ...// } finally {// ringBuffer.publish(sequence);// }//@return The next sequence to publish to.@Overridepublic long next() {return sequencer.next();}//Publish the specified sequence.//This action marks this particular message as being available to be read.//@param sequence the sequence to publish.@Overridepublic void publish(long sequence) {sequencer.publish(sequence);}//Get the event for a given sequence in the RingBuffer.//This call has 2 uses. //Firstly use this call when publishing to a ring buffer.//After calling RingBuffer#next() use this call to get hold of the preallocated event to fill with data before calling RingBuffer#publish(long).//Secondly use this call when consuming data from the ring buffer. //After calling SequenceBarrier#waitFor(long) call this method with any value greater than that //your current consumer sequence and less than or equal to the value returned from the SequenceBarrier#waitFor(long) method.//@param sequence for the event//@return the event for the given sequence@Overridepublic E get(long sequence) {//调用父类RingBufferFields的elementAt()方法return elementAt(sequence);}...
}abstract class RingBufferPad {protected long p1, p2, p3, p4, p5, p6, p7;
}abstract class RingBufferFields<E> extends RingBufferPad {...private static final Unsafe UNSAFE = Util.getUnsafe();private final long indexMask;//环形数组存储事件消息private final Object[] entries;protected final int bufferSize;//RingBuffer的sequencer属性代表了当前线程对应的生产者protected final Sequencer sequencer;RingBufferFields(EventFactory<E> eventFactory, Sequencer sequencer) {this.sequencer = sequencer;this.bufferSize = sequencer.getBufferSize();if (bufferSize < 1) {throw new IllegalArgumentException("bufferSize must not be less than 1");}if (Integer.bitCount(bufferSize) != 1) {throw new IllegalArgumentException("bufferSize must be a power of 2");}this.indexMask = bufferSize - 1;//初始化数组this.entries = new Object[sequencer.getBufferSize() + 2 * BUFFER_PAD];//内存预加载fill(eventFactory);}private void fill(EventFactory<E> eventFactory) {for (int i = 0; i < bufferSize; i++) {entries[BUFFER_PAD + i] = eventFactory.newInstance();}}protected final E elementAt(long sequence) {return (E) UNSAFE.getObject(entries, REF_ARRAY_BASE + ((sequence & indexMask) << REF_ELEMENT_SHIFT));}...
}public abstract class AbstractSequencer implements Sequencer {private static final AtomicReferenceFieldUpdater<AbstractSequencer, Sequence[]> SEQUENCE_UPDATER =AtomicReferenceFieldUpdater.newUpdater(AbstractSequencer.class, Sequence[].class, "gatingSequences");//环形数组的大小protected final int bufferSize;//等待策略protected final WaitStrategy waitStrategy;//当前生产者的进度protected final Sequence cursor = new Sequence(Sequencer.INITIAL_CURSOR_VALUE);//每一个Sequence都对应着一个消费者(一个EventHandler或者一个WorkHandler)//这些Sequence会通过SEQUENCE_UPDATER在执行Disruptor的handleEventsWith()等方法时，//由RingBuffer的addGatingSequences()方法进行添加protected volatile Sequence[] gatingSequences = new Sequence[0];...//Create with the specified buffer size and wait strategy.//@param bufferSize The total number of entries, must be a positive power of 2.//@param waitStrategypublic AbstractSequencer(int bufferSize, WaitStrategy waitStrategy) {if (bufferSize < 1) {throw new IllegalArgumentException("bufferSize must not be less than 1");}if (Integer.bitCount(bufferSize) != 1) {throw new IllegalArgumentException("bufferSize must be a power of 2");}this.bufferSize = bufferSize;this.waitStrategy = waitStrategy;}...
}abstract class SingleProducerSequencerPad extends AbstractSequencer {protected long p1, p2, p3, p4, p5, p6, p7;public SingleProducerSequencerPad(int bufferSize, WaitStrategy waitStrategy) {super(bufferSize, waitStrategy);}
}abstract class SingleProducerSequencerFields extends SingleProducerSequencerPad {public SingleProducerSequencerFields(int bufferSize, WaitStrategy waitStrategy) {super(bufferSize, waitStrategy);}//表示生产者的当前序号，值为-1protected long nextValue = Sequence.INITIAL_VALUE;//表示消费者的最小序号，值为-1protected long cachedValue = Sequence.INITIAL_VALUE;
}public final class SingleProducerSequencer extends SingleProducerSequencerFields {protected long p1, p2, p3, p4, p5, p6, p7;//Construct a Sequencer with the selected wait strategy and buffer size.//@param bufferSize the size of the buffer that this will sequence over.//@param waitStrategy for those waiting on sequences.public SingleProducerSequencer(int bufferSize, WaitStrategy waitStrategy) {super(bufferSize, waitStrategy);}...@Overridepublic long next() {return next(1);}@Overridepublic long next(int n) {//Sequence的初始化值为-1if (n < 1) {throw new IllegalArgumentException("n must be > 0");}//nextValue指的是当前Sequence//this.nextValue为SingleProducerSequencerFields的变量//第一次调用next()方法时，nextValue = -1//第二次调用next()方法时，nextValue = 0//第三次调用next()方法时，nextValue = 1//第四次调用next()方法时，nextValue = 2//第五次调用next()方法时，nextValue = 3long nextValue = this.nextValue;//第一次调用next()方法时，nextSequence = -1 + 1 = 0//第二次调用next()方法时，nextSequence = 0 + 1 = 1//第三次调用next()方法时，nextSequence = 1 + 1 = 2//第四次调用next()方法时，nextSequence = 2 + 1 = 3//第五次调用next()方法时，nextSequence = 3 + 1 = 4long nextSequence = nextValue + n;//wrapPoint会用来判断生产者序号是否绕过RingBuffer的环//如果wrapPoint是负数，则表示还没绕过RingBuffer的环//如果wrapPoint是非负数，则表示已经绕过RingBuffer的环//假设bufferSize = 3，那么：//第一次调用next()方法时，wrapPoint = 0 - 3 = -3，还没绕过RingBuffer的环//第二次调用next()方法时，wrapPoint = 1 - 3 = -2，还没绕过RingBuffer的环//第三次调用next()方法时，wrapPoint = 2 - 3 = -1，还没绕过RingBuffer的环//第四次调用next()方法时，wrapPoint = 3 - 3 = 0，已经绕过RingBuffer的环//第五次调用next()方法时，wrapPoint = 4 - 3 = 1，已经绕过RingBuffer的环long wrapPoint = nextSequence - bufferSize;//cachedGatingSequence是用来将消费者的最小消费序号缓存起来//这样就不用每次执行next()方法都要去获取消费者的最小消费序号//第一次调用next()方法时，cachedGatingSequence = -1//第二次调用next()方法时，cachedGatingSequence = -1//第三次调用next()方法时，cachedGatingSequence = -1//第四次调用next()方法时，cachedGatingSequence = -1//第五次调用next()方法时，cachedGatingSequence = 1long cachedGatingSequence = this.cachedValue;//第四次调用next()方法时，wrapPoint大于cachedGatingSequence，执行条件中的逻辑if (wrapPoint > cachedGatingSequence || cachedGatingSequence > nextValue) {//最小的消费者序号long minSequence;//自旋操作：//Util.getMinimumSequence(gatingSequences, nextValue)的含义就是找到消费者中最小的序号值//如果wrapPoint > 消费者中最小的序号，那么生产者线程就需要进行阻塞//即如果生产者序号 > 消费者中最小的序号，那么就挂起并进行自旋操作//第四次调用next()方法时，nextValue = 2，wrapPoint = 0，gatingSequences里面的消费者序号如果还没消费(即-1)，则要挂起while (wrapPoint > (minSequence = Util.getMinimumSequence(gatingSequences, nextValue))) {//TODO: Use waitStrategy to spin? LockSupport.parkNanos(1L); }//cachedValue接收了消费者的最小序号//第四次调用next()方法时，假设消费者的最小序号minSequence为1，则cachedValue = 1this.cachedValue = minSequence;}//第一次调用完next()方法时，nextValue会变为0//第二次调用完next()方法时，nextValue会变为1//第三次调用完next()方法时，nextValue会变为2//第四次调用完next()方法时，nextValue会变为3//第五次调用完next()方法时，nextValue会变为4this.nextValue = nextSequence;//第一次调用next()方法时，返回的nextSequence = 0//第二次调用next()方法时，返回的nextSequence = 1//第三次调用next()方法时，返回的nextSequence = 2//第四次调用next()方法时，返回的nextSequence = 3//第五次调用next()方法时，返回的nextSequence = 4return nextSequence;}@Overridepublic void publish(long sequence) {//设置当前生产者的sequencecursor.set(sequence);//通过等待策略通知阻塞的消费者waitStrategy.signalAllWhenBlocking();}...
}public final class Util {...//Get the minimum sequence from an array of {@link com.lmax.disruptor.Sequence}s.//@param sequences to compare.//@param minimum an initial default minimum. If the array is empty this value will be returned.//@return the smaller of minimum sequence value found in sequences and minimum; minimum if sequences is emptypublic static long getMinimumSequence(final Sequence[] sequences, long minimum) {for (int i = 0, n = sequences.length; i < n; i++) {long value = sequences[i].get();minimum = Math.min(minimum, value);}return minimum;}...
}public final class MultiProducerSequencer extends AbstractSequencer {...@Overridepublic long next() {return next(1);}@Overridepublic long next(int n) {if (n < 1) {throw new IllegalArgumentException("n must be > 0");}long current;long next;do {//获取当前生产者的序号current = cursor.get();next = current + n;//wrapPoint会用来判断生产者序号是否绕过RingBuffer的环//如果wrapPoint是负数，则表示还没绕过RingBuffer的环//如果wrapPoint是非负数，则表示已经绕过RingBuffer的环long wrapPoint = next - bufferSize;//cachedGatingSequence是用来将消费者的最小消费序号缓存起来//这样就不用每次执行next()方法都要去获取消费者的最小消费序号long cachedGatingSequence = gatingSequenceCache.get();if (wrapPoint > cachedGatingSequence || cachedGatingSequence > current) {//gatingSequence表示的是消费者的最小序号long gatingSequence = Util.getMinimumSequence(gatingSequences, current);if (wrapPoint > gatingSequence) {//TODO, should we spin based on the wait strategy?LockSupport.parkNanos(1); continue;}gatingSequenceCache.set(gatingSequence);} else if (cursor.compareAndSet(current, next)) {break;}} while (true);return next;}...
}文章转载自：东阳马生架构
原文链接：Disruptor—3.核心源码实现分析 - 东阳马生架构 - 博客园
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